Voltage converters may be employed in a variety of audio applications, including, for example, an RF power amplifier driving an antenna of a mobile device. Traditionally, power supply circuits of the RF power amplifiers are directly connected to a battery of the mobile device. However, this approach wastes a significant amount of energy and reduces battery life of the mobile device. For example, when the mobile device is in close proximity to a base station, only a fraction of a maximum power supplied by the battery is required to ensure reliable wireless voice and data communication. When the mobile device is further away from the base station, the battery may be required to supply the maximum available power to sustain a reliable wireless communication link. To maximize energy efficiency and battery life, a voltage converter capable of adjusting its output voltage may be used.
The voltage converter may be implemented, for example, as a DC-DC converter that generates a regulated dc output voltage which may be a fraction of its unregulated input dc voltage. A boost DC-DC converter in a steady state provides a regulated dc output voltage, which is higher than an unregulated input dc voltage. On the other hand, a buck DC-DC converter in a steady state provides a regulated dc output voltage, which is lower than an input dc voltage. Load perturbations or changes in the unregulated dc input voltage may cause a DC-DC converter to exhibit an output voltage ripple. To reduce the output voltage ripple, a dc-dc converter with wide bandwidth is typically needed. Unfortunately, as the bandwidth becomes wider, the dc-dc converter becomes less stable.
To design a DC-DC converter with good bandwidth and stability, a compensator may be used to regulate the output dc voltage of a DC-DC converter. The compensator compares the output voltage to a voltage reference to generate an error signal which subsequently determines the duty cycle of the pulse-width modulated signal provided by a modulator. The compensator may hold the output voltage constant by controlling the duty cycle of the pulse-width modulated signal. In a closed loop configuration, the DC-DC converter will reach a stable point of equilibrium as the output voltage approaches the reference voltage provided by a reference voltage source.
Conventional compensators are often implemented as a Type I, Type II, or Type III compensation network as illustrated in FIGS. 6A-C, respectively. A conventional Type I compensator is an integrator amplifier having a transfer function with a single pole within its frequency bandwidth at the origin. A conventional Type II compensator introduces an additional pole and a zero to shape the phase and gain response of the feedback connected voltage converter. A conventional Type III compensator uses two zeros, a pole at the origin and two additional poles to provide a phase boost and further increases the bandwidth of the voltage converter. However, these conventional approaches flatten the gain in order to make the overall closed loop voltage converter system stable.
There is, therefore, a continued need for compensators that provide high gains and a minimal output voltage overshoot in the band of interest while satisfying the stability criteria of the overall closed loop system.